The present invention relates generally to a silicon emitter with a contact layer of low porosity porous silicon material including a heavily doped region and to a method of fabricating a silicon emitter with a contact layer of low porosity porous silicon material including a doped region. More specifically, the present invention relates to a silicon emitter including a contact layer of low porosity porous silicon material including a heavily doped region for reducing contact resistance between an active layer of high porosity porous silicon material and a top electrode and for increasing electron emission efficiency and emission stability of the top electrode and to a method of fabricating the same.
FIG. 1 illustrates a prior porous silicon emitter 100. The prior porous silicon emitter 100 is a diode structure that includes a heavily doped n+ silicon (Si) substrate 103 that serves as an electron injection layer, an optional ohmic contact 105 in electrical contact with the substrate 103, an active porous silicon (Si) layer 101 formed on the substrate 103, and an electrode 107 formed on the active porous silicon layer 101 and in electrical communication with the electrode 107. When the electrode 107 is biased positively relative to the substrate 103, a diode current Id, supplied by a voltage source V1, passes through the active layer 101 and the substrate 103. A fraction of the diode current Ie, is injected into a vacuum region (not shown) above the electrode 107 and is collected by a collector electrode 115 that is positioned opposite the electrode 107. The collector electrode 115 is biased positively relative to the electrode 107 by a voltage source V2 to extract electrons exe2x88x92 that are emitted by the electrode 107. The electrodes (107, 115) and the ohmic contact 105 can be made from an electrically conductive material such as gold (Au) or aluminum (Al).
One disadvantage of the prior porous silicon emitter 100 is that the active porous silicon (Si) layer 101 has a high porosity that results in a high series contact resistance Rc between the electrode 107 and the active porous silicon (Si) layer 101. The resistance Rc is comparable with or even larger than the resistance of the active porous silicon (Si) layer 101 at high voltage. Consequently, the high series contact resistance Rc creates an undesirable/unintentional voltage drop between the active layer 101 and the electrode 107 that reduces an electron emission efficiency of the porous silicon emitter 100.
Moreover, the high series contact resistance Rc results in a higher power consumption and higher power dissipation (waste heat). This tends to reduce the useful life time of the emitter 100. In battery powered applications it is desirable to reduce power consumption so that battery life and operating time are extended. Furthermore, it is desirable to reduce the amount of waste heat generated by a system because thermal management systems such as fans and heat sinks add to system cost, weight, and complexity.
A second disadvantage of the prior porous silicon emitter 100 is that the contact resistance Rc causes the diode and emission current to saturate at high bias voltages supplied by V1. It is desirable to have the electron emission current increase with increasing voltage levels. However, if saturation occurs the electron emission current peaks and does not increase with increasing voltage.
Finally, another disadvantage of the prior porous silicon emitter 100 is that the active porous silicon (Si) layer 101 has a high contact resistance with the electrode 107 that results in a reduction in electron emission efficiency.
Therefore, there exists a need for a porous silicon emitter that reduces the series contact resistance between an active porous silicon layer and an electrode of the porous silicon emitter. There is also a need for a porous silicon emitter that can operate at lower voltages thereby reducing power consumption and generation of waste heat. Furthermore there is a need for a porous silicon emitter that does not saturate at higher voltages so that high emission currents and efficiency are obtainable at those higher voltages.
The present invention solves the aforementioned problems created by the high series contact resistance by including a contact layer of low porosity and low resistivity porous silicon material between an active layer of high porosity porous silicon material and a top electrode. Furthermore, a portion of the contact layer of low porosity porous silicon that is adjacent to the top electrode includes a heavily doped region resulting in an increased electron emission efficiency and emission current from the top electrode and a further reduction of the operating voltage. The contact layer of low porosity porous silicon reduces the series contact resistance between the top electrode and the active layer of high porosity porous silicon. As a result, when a bias voltage is applied to the diode, the voltage drop between the active layer and the top electrode is reduced, and most of the voltage drop is produced in the active layer.
Additionally, the aforementioned problems associated with high power consumption and high power dissipation of the prior porous silicon emitter are solved by the contact layer of low porosity porous of the present invention because the reduced contact resistance results in reduced power consumption and reduced power dissipation. Furthermore, the reduced contact resistance allows for operation of the electron emitter at reduced voltage levels that are commensurate with the goals of low power consumption and low power dissipation.
Broadly, the present invention is embodied in a high emission electron emitter and a method of fabricating a high emission electron emitter. A high emission electron emitter according to the present invention includes an electron injection layer, an active layer of high porosity porous silicon material in contact with the electron injection layer, a contact layer of low porosity porous silicon material in contact with the active layer and including a heavily doped region that extends inward of an interface surface of the contact layer, and a top electrode in contact with the interface surface of the contact layer. The contact layer with the heavily doped region reduces contact resistance between the active layer and the top electrode. The doped region reduces the resistivity of the contact layer. The electron injection layer is made from an electrically conductive material such as an n+ semiconductor, n+ single crystal silicon (Si), a silicide, a metal, or a layer of metal on a glass substrate. The active layer and the contact layer can be formed in an epitaxial layer of silicon (Si), a polysilicon layer of silicon (Si), a layer of amorphous silicon (Si), or a layer of silicon carbide (SiC) that is deposited on the electron injection layer. The top electrode is an electrically conductive material such as gold (Au) or aluminum (Al).
A method of fabricating a high emission electron emitter includes doping an interface surface of a layer of silicon material with a n+ dopant, annealing the layer of silicon material to form a doped region that extends inward of an interface surface of the layer of silicon material, electrochemically anodizing the interface surface in a hydrofluoric acid (HF) solution in either one of a dark ambient or an illuminated ambient at a first anodization current density to form a contact layer of low porosity porous silicon material. The first anodization current density is maintained for a first period of time until the contact layer has reached a first thickness. Next, the first anodization current density is increased to a second anodization current density (i.e. the second anodization current density is greater than or equal to the first anodization current density) to form an active layer of high porosity porous silicon material. The second anodization current density is maintained for a second period of time until the active layer has reached a second thickness. Finally, an optional top electrode can be deposited on the interface surface.
In one embodiment of the present invention, the electron injection layer comprises a material including but not limited to a n+ semiconductor, n+ single crystal silicon, a metal, metallic alloys, a layer of metal on a glass substrate, and suicides of metal.
In another embodiment of the present invention, the contact layer of low porosity porous silicon material and the active layer of high porosity porous silicon material can be a material including but not limited to porous epitaxial silicon, porous polysilicon, and porous silicon carbide.
In alternative embodiments of the present invention, the porous epitaxial silicon can be: intrinsic porous epitaxial silicon; nxe2x88x92 porous epitaxial silicon; or pxe2x88x92 porous epitaxial silicon. The porous polysilicon can be: intrinsic porous polysilicon; nxe2x88x92 porous polysilicon; or pxe2x88x92 porous polysilicon.
In yet another embodiment of the present invention, the n+ doped region is doped using a process including but not limited to ion implantation, diffusion, and insitu deposition. The heavily doped region can include but is not limited to n-type dopants such as arsenic, antimony, phosphorus, vanadium, and nitrogen.
In one embodiment of the present invention, the electron injection layer includes an ohmic contact.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.